Electrically conductive polymeric materials may be implemented into applications such as electronic textiles, robotics, EMI shielding, and lightning strike protections. However, traditional methods of obtaining conductivity in polymeric materials, such as using inherently conductive polymers or adding conductive fillers to a polymer matrix, can be difficult to process, due to high melt temperatures and high viscosity, respectively. In addition, typical approaches for making electrically or thermally conductive polymers are focused on optimizing electrical and thermal transport and do not offer the ability to independently tune mechanical and electrical/thermal response.
One method of obtaining electrical conductivity in polymeric materials is to use inherently electrically conductive polymers such as polythiophene and polyanilene. However, most inherently electrically conductive polymers are rigid polymers with a high melt temperature and melt viscosity that are difficult to process.
Another method of obtaining electrical conductivity in polymeric materials is to add electrically conductive particulate fillers to a polymer. However, obtaining conductivity in polymeric materials through this method typically requires a high concentration (loading) of conductive particulate fillers. Adjacent filler particles must be in contact with or in close enough proximity to one another to create an electrically conductive pathway through the polymer. The threshold concentration of an electrically conductive particulate filler where an electrically conductive pathway is first formed is called the electrical percolation threshold. The high particulate filler concentrations required to obtain electrical conductivity in the polymer will cause the viscosity of the polymer composite to rise exponentially, thereby decreasing the ability to process the polymeric material. In addition, the particulate fillers will increase the modulus and decrease the elongation at break of the material at particulate loadings lower than those required to achieve electrical percolation.
While the electrical percolation threshold can be decreased by using high aspect ratio particulates, the processing of the polymer is still influenced by the mechanical percolation threshold which occurs at lower particulate loadings than the electrical percolation threshold regardless of particle geometry. The mechanical percolation threshold is the concentration of particulate in the polymer at or above which the particulates begin to impact the motion of adjacent particles, thereby affecting the mechanical properties of the polymer, such as viscosity and elasticity. With low filler particulate loading, the particulates are in a dilute regime where adjacent particulates do not influence each other and the viscosity is similar to the host polymer, typically increasing in a linear fashion with increasing filler loading. As the particulate filler concentration is further increased, the polymer-particle interfacial regions between adjacent particulate overlap, impacting the motion of adjacent particles, and producing a dramatic increase in the viscosity, which is non-linear with increasing filler loading. To obtain electrical conductivity, the particulate loading must be further increased into a concentrated regime where the particles are in direct contact with one another or the inter-particle spacing is very small. This results in an exponential increase in the viscosity. As a result, it is very difficult to process most electrically conductive polymer composites regardless of filler geometry. Similar to viscosity, the modulus of the particulate loaded polymer increases with increasing particulate loading. Therefore it is difficult to produce a flexible polymer that is conductive with a targeted mechanical stiffness.
The incorporation of thermally conductive particulate fillers into a polymer to obtain thermal conductivity suffers from the same drawbacks as obtaining electrical conductivity in a polymer composite by incorporating electrically conductive particulates including difficulty processing using traditional processing techniques, increased modulus, and decreased elongation at break.
Most electrically conductive particulate fillers are also thermally conductive however, several thermally conductive particulate are not electrically conductive.
Therefore, the inventors have provided improved flexible, electrically and/or thermally conductive polymer composites and methods of preparing such flexible, electrically and/or thermally conductive polymer composites, with an ability to simultaneously control mechanical flexibility in addition to electrical and/or thermal conductivity.